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1. Field of the Invention
The present invention generally relates to sensing roadway markers for driver assistance and vehicle control, and more particularly to an intelligent sensing system for sensing roadway markers for driver assistance and vehicle control.
2. Description of the Background Art
Over the years, numerous systems have been developed to provide automatic vehicle control to provide hands-free and feet-free driving of automobiles. These systems automate either steering control (referred to as lateral control), throttle and/or brake control (longitudinal control), or all of the above (complete vehicle automatic control, also referred to as an Automated Highway System or AHS).
Longitudinal control typically relies on some form of forward-looking sensor, e.g. radar, to provide collision warning and/or distance to preceding vehicles, and takes the form of either adaptive cruise control or platooning, wherein vehicles are formed into electronically coupled groups, similar to a train, but without the mechanical coupling. Lateral control relies on a variety of sensors to determine the vehicle""s lateral offset from the lane centerline, and usually an estimate of upcoming roadway geometry, e.g. curvature.
Sensors for lateral control can be based infrastructure-based or infrastructure-independent. Examples of infrastructure-based systems include systems based on detection of embedded infrastructure, such as discrete magnetic reference markers or continuous magnetic tape. Infrastructure-independent methods include vision-based sensing and Global Positioning System (GPS) sensing; however, these systems rely on infrastructure in the sense of reliable roadway markings in the former case, and a reliable and accurate roadway Geographical Information System (GIS) database in the latter.
In recent years, systems have been developed using similar sensing technologies, but with the purpose of providing visual, audible, or tactile feedback and warning (i.e. driver assistance) to the driver of the vehicle, thus enhancing the driver""s ability to operate the vehicle in degraded visibility conditions (e.g. in dense fog or snow-induced whiteout conditions). For these types of conditions, visual sensing for lateral control is not the ideal primary sensing system, as the performance of these systems is degraded in poor optical conditions. Infrastructure-based systems appear to provide the most reliable and robust solution for these conditions, particularly in areas that may be subject to satellite obscuration (e.g., mountainous regions). These systems also must include a forward collision warning system, as they are intended to enable the vehicle operator to drive in degraded visibility conditions, and must provide sufficient warning of upcoming obstacles, in order to protect this vehicle""s driver, as well as others on the roadway.
There are several well-developed technologies for vehicle lateral guidance. They may be classified as vision-based, roadway reference system based, and radio wave signal based methods. Vision-based or other optical systems are generally considered inappropriate in poor visibility conditions such as fog, rain, and particularly snow. Roadway reference systems include induction wires, radar-reflective tape, magnetic tape, and discrete reference markers. Reference systems may be passive or active elements. Wire-guided vehicle control represents one active system; construction and maintenance issues preclude its use in a highway environment. Example markers include magnets, colored paint marks, retroreflective raised pavement markers, and radar-reflective materials. However, any optic-based marker detection system faces the same problem as any other vision-based system in low visibility environments; as such, these systems are not feasible here. Magnetic markers and overhead induction wires are possible all weather solutions for lateral guidance.
Magnetic markers for lateral control have been found to have a maximum lateral sensing error of 1.5 cm with 1 cm standard deviation, which is well within the 3 cm needed for commercially viable systems. Discrete magnetic markers embedded in the roadway can be used for longitudinal position measurement as well as lateral control. Moreover, magnetic markers can be coded with other roadway information, which each vehicle can read via onboard magnetometers. Each magnet is capable of storing one bit of information. The coded sequence consists of header code to initialize and uniquely identify the message, followed by the roadway information. Error detection codes can be placed at the end of the message as well. Magnetic pavement marker tape has been shown to have similar performance for lateral position measurement. However, it cannot be coded to provide roadway information. Furthermore, retrofitting magnetic tape to current highways may be difficult, and possibly more costly than installing magnetic markers; further study is warranted here. Nonetheless, concern has been expressed that magnetic markers may lead to temperature induced stress concentration and faster road deterioration. However, the maturity and robustness of the magnetic marker technology warrants its use in the current development for snowplow guidance. On the other hand, experience gained in recent research indicated other deficiencies in the state of the art in the magnetic sensing system.
For example, the use of continuous magnetic marking material to provide vehicle guidance and control is well known. However, such systems cannot provide information coding, and thus cannot provide upcoming roadway geometry (e.g. curvature), or other infrastructure information (e.g. upcoming bridge abutments). In addition, the algorithms used to detect the magnetic xe2x80x9ctapexe2x80x9d and determine lateral offset are typically based on the use of frequency and phase information, which assists in separating signal from noise, but cannot provide lateral offset down to zero vehicle speed. Furthermore, these systems appear to be limited to vehicle speeds of about five MPH and above. In addition, since these systems typically use a square-wave magnetization pattern to support the signal processing, at least one wavelength must be detected before lateral offset can be obtained; this introduces a signal processing delay of xc2xd wavelength.
Systems that employ discrete analog magnetometers suffer from a number of deficiencies as well. For example, information from the sensors (magnetic field strength) is transmitted to a central computer""s data acquisition system over analog lines. This introduces serious noise issues in a vehicle environment. In addition, due to the large number of sensors and channels required, the conductor and part count is very high, leading to difficulties in installation, maintenance, and trouble-shooting. The high channel count also introduces the need for custom data acquisition and signal conditioning boards, further aggravating the problem. These and other issues make it difficult to protect subsystems against harsh environments (e.g., snow, ice, and salt). In addition, some of the algorithms used are subject to a number of problems. For example, typical algorithms rely on vehicle-specific calibration, and the calibration table is magnet-type or magnet-strength dependent, so that different calibration tables must be used for different areas of the roadway. While standard strength magnets are used to code much of the roadway, smaller yet much higher strength magnets are used to code bridges and other structures. In addition, the implementation uses fixed gain sampling, which reduces effective use of sensor range, and introduces unnecessary saturation. Furthermore, these algorithms are based on peak detection and, therefore, cannot operate down to zero vehicle speed. This creates significant problems at low speeds. In addition, vehicle lateral offset is provided only at instances of peak detection (i.e. when the vehicle""s sensor system is directly over the magnets as it travels along the road). This means that the sensor update rate is dictated by magnet spacing combined with vehicle speed. This update rate should be independent of these factors, and should only be dictated by the requirements of the control or driver assistance system. In addition, such systems do not lend themselves to miniaturization in their current form, as they require a centralized industrial computer, custom I/O boards, and the resultant relatively large supporting power and mechanical infrastructure. Furthermore, these systems tend to lack the modularity, robustness, and maintainability necessary for a deployed commercial system.
Thus, it is desirable to have a lateral sensing system that uses digital transmission for safety-critical data, such as lateral offset, magnet field strength data, and other critical sensing information. Digital transmission resolves the vehicle noise issue, as well as reducing cabling requirements and easing installation, diagnosis, and maintenance. The resultant system will also be more robust to harsh environmental conditions. In addition, it is desirable, in the case of magnetic material sensing, to minimize or remove sensitivity to absolute magnetic field strength as well as sensor height. A system that avoids use of peak detection is also desirable, in that it allows the sampling rate to be selected based on the requirements of the control system, independent of magnetic marker spacing and vehicle speed. By avoiding peak detection, and appropriately designing the overall system, it is also possible to develop a sensor that can perform down to zero speed, and over the full range of possible vehicle operating speeds. Algorithms used in the system should be insensitive to magnet strength variation, ride height variation, and vehicle speed. Also, the system should limit exposure to sensor saturation while maximizing the available sensing range and maximizing use of available data representation. In addition, a system that can support both discrete and continuous magnetic reference systems will be of broad appeal for end users and infrastructure providers. The present invention satisfies those needs, as well as others, and overcomes deficiencies in current technology.
The present invention generally comprises methods and apparatus to sense both discrete and continuous magnetic reference systems installed in the roadway, and provide information to support lateral and, to some extent if necessary, longitudinal and vertical vehicle control and/or driver assistance. More particularly, the invention comprises an intelligent sensing architecture and an assortment of methods for deriving lateral (across road), longitudinal (along road) and vertical offset (above road), and other pertinent information, from both continuous and discrete magnetic reference systems.
By way of example, and not of limitation, the invention comprises a method and apparatus for determining the position of an object relative to a magnetic reference marker by sensing, with a sensor associated with the object, at least one axial field strength component of the magnetic field emitted from the magnetic reference, computing a ratio of the sensed axial field strength components, and then determining the positional offset of the object from said magnetic reference as a function of the ratio. Preferably, the magnetic reference comprises an infrastructure that defines a roadway reference line, and the positional offset comprises lateral offset from the roadway reference line.
The ratio provides a one-to-one mapping of the sensor outputs to lateral offset. The output of this mapping for selected values of X (longitudinal offset) yields a surface that can be sliced for different values of vertical offset Z to produce a domain map from which the lateral offset can be determined. The lateral offset, which is independent of the magnetic reference field strength due to the use of the ratio approach, is then determined from the domain map. Points along the domain map which are between sample points can be determined by interpolation or curve fitting. These functions are carried out preferably using a digital data processor and associated programming.
The invention is usable over a wide range of vehicle operating speeds, such as from zero to 150 MPH. The signal processing methods of the present invention are insensitive to magnet field strength variation, vehicle ride height variation, and vehicle speed. Embodiments of the invention provide noise immunity, robustness, manufacturability, portability, low cost, superior environmental operation, modularity, compactness, installation, diagnostics, and maintenance.
The invention can be used for a wide variety of applications, including, but not limited to, vehicle guidance and control, driver assistance, robotics, position determination of objects, location finding, depth finding, and other applications where relative position from a magnetic reference point is useful. Additionally, the invention is useable in a wide range of operating environments, including those subject to severely degraded visibility and harsh environmental operating conditions, such as blizzard, fog, and whiteout conditions.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.